This invention relates to digital time base correctors and more particularly to velocity compensation in time base correctors.
Time base correctors are used for example with video tape recorders (VTR) to correct errors in timing of the signal produced at the output of the VTR with reference to a stable synchronizing signal. These errors arise because of changes in the velocity of the tape transport and the scanning mechanism of the VTR.
Time base correctors may also include a compensator for reinserting information lost on the tape due to oxide dropout (see for example British Patent No. 1,436,757 and U.s. Pat. No. 3,949,416).
In known time base correctors, the incoming video signal from the VTR is converted into a digital format by an analogue-to-digital converter (ADC). The digital output from the ADC is stored in a store which will conveniently have a capacity sufficient to store one or more complete television lines. The output from the store is converted into an analogue video signal by a digital-to-analogue converter (DAC).
The incoming video signal is converted into digital format at a clock rate determined by the input oscillator. The input oscillator frequency is caused to follow the timing perturbations of the output signal of the VTR.
The information read from the store into the DAC is under the control of an output oscillator which is generally locked to a local synchronizing signal and therefore is at a fixed frequency.
The principle of the operation is that information is written into the store at a speed which depends upon the perturbations on the output from the VTR but that the information is read out of the store at a constant rate. Thus, the equipment eliminates time perturbations occurring on the video signal.
In one known configuration, the input oscillator designed to track the information coming from the VTR takes the form of a phase locked loop (PLL). In such a system a voltage controlled oscillator (VCO) running at a suitable sampling frequency produces an output which is used to drive the ADC. The output is also applied to a system of multipliers and dividers which reduces the frequency to precisely that of the television line frequency. The output of the system is applied to one input of a phase discriminator which has its other input coming from the separated line synchronizing pulse from the VTR.
The phase discriminator produces a variable voltage at its output which is arranged to control the frequency of the voltage controlled oscillator in a manner which will cause the phase error at the two inputs to the discriminator to be close to zero.
However, with such a system the instantaneous frequency of the VCO is a function of the output of the phase discriminator over several preceding lines. Therefore, when a phase error between the off-tape signal and the VCO occurs, there is a finite response time before the error can be reduced to zero. For some types of off-tape timing perturbations it is impossible to design a phase locked loop which satisfactorily tracks the off-tape signal (that is to a close enough timing accuracy).
In the corrector disclosed in our co-pending cognate British Patent Applications 11793/74 and 11794/74 (U.S. Pat. Nos. 3,971,063 and 3,978,519), more accurate compensation of the timing errors is achieved by providing a triggered oscillator which is triggered in response to the incoming sync pulse alone or colour burst and sync pulse signals. However, as the frequency error is corrected on the next incoming line the error correction is always one line late.